This invention provides a process to produce a polar group containing polymer wherein the polar group containing polymer has a narrow molecular weight distribution.
Polar group containing polymers may be prepared by copolymerizing monomers such as methacrylates and nitriles, with alpha olefin monomers, but such copolymers are generally prepared by free radical polymerization. Free radical polymerization is subject to premature termination by various mechanisms and therefore results in a very wide molecular weight distribution. A polymer with a wide molecular weight distribution is undesirable for many end uses because the low molecular weight portions impart undesirable properties of low molecular weight polymers, such as low glass transition temperatures, low tensile strengths, and low melt flow temperatures, while the high molecular weight portion imparts many undesirable properties of high molecular weight polymers, such as high melt viscosity. Further, free radical mechanisms cannot be utilized to produce block copolymers. Block copolymers of styrene and conjugated diolefins are useful in many applications due to their ability to phase separate into domains of each type of block. The benefits of phase separatable polymeric blocks within the polymer molecules cannot be realized with free radical polymerized polymers. As opposed to free radical polymerization, anionic polymerization can result in polymers with very narrow molecular weight distributions and sequential addition of monomers can result in distinct polymeric blocks. Unfortunately, polar monomers are generally protic, and terminate anionic polymerization. Polar monomers can therefore generally not be copolymerized with other monomers to form polar group containing polymers with narrow molecular weight distributions.
Polymers containing polar functional groups, including block copolymers, can be produced by extruder grafting alpha-beta unsaturated polar group containing monomers to base polymers. Maleic anhydride and maleic acid are the most commonly grafted monomers. A process utilizing a free radical initiator is typically utilized, such as the process disclosed in U.S. Pat. No. 4,657,971. This grafting is easily accomplished in an extruder. A polymer with a narrow molecular weight distribution may be used as the base material, but the free radical grafting mechanism results in considerable scissioning and coupling of the base polymer. This results in a polar group containing polymer which has about 20 weight percent of the polymer either degraded or coupled.
When a styrene-conjugated diolefin block copolymer is extruder grafted with an ethylenically unsaturated monomer, the grafting will occur predominantly within the conjugated diolefin block. In many applications, it is desirable to have at least a portion of the functionality grafted within the styrene blocks.
Processes to graft polar functional groups to polystyrene blocks of block copolymers of styrene and conjugated diolefins are also known, but are also replete with shortcomings. One of these processes involves reacting the polymer with a metal alkyl, and then replacing the metal alkyl with an electrophile such as carbon dioxide, ethylene oxide, aldehydes, ketones, carboxylic acids, salts, epoxides and isocyanates. Such a process is disclosed in U.S. Pat. No. 4,797,447. This process is carried out in a solution. A process which can be accomplished in a melt phase is desirable due to the expense and process steps involved in dissolving the polymer and then removing the solvent. Additionally, like free radical grafting, this process results in considerable scissioning, coupling and degradation of the base polymer.
It is therefore an object of this invention to provide a process to produce a polar group containing polymer wherein the polar group containing polymer is not excessively coupled or degraded and wherein the process is carried out in a melt. In another aspect, it is an object to provide the product of this process.